Noobs who try to raise their stats and levels by boosting in 1 on 1 matches. They also flat out suck and are nerds, who focus on getting achievements more than playing the game.
"Hey Larry, I think that guy in game chat called us Noob Boosters? What does that mean?"
"He's just jealous because we have sweet stats and are fifth prestige, even though our KDR is .03."